1. Field of the Invention
The invention is directed to a plate-shaped piezoactuator assembly and to a method for manufacturing such an assembly, particularly for ink printer heads that are assembled of ink printer modules in stacked fashion.
2. Description of the Prior Art
Ink printer heads assembled of stacked ink printer modules are employed in small, fast printers that are in turn a component of moderate machines for franking postal matter or for printing addresses. Differing from standard office printers having line-by-line imprinting, printing in these machines ensues as a one-time franking impression in one pass of the postal matter. Corresponding to this significantly greater print width --approximately one inch--, the number of ink nozzles to be arranged under one another, and thus the number of piezoactuators in an ink printer head, is substantially larger than in the case of ink printer heads for office printers. In order to satisfy the current customer desire to print blocks with words as well as image characters in a postage meter machine with good print quality, printer resolutions of approximately 200 dpi are required, which means ink printer heads having the same number of nozzles and piezoactuators given a printing width of one inch. Of necessity, such ink printer heads are implemented in a planar or stacked structure; first, for reasons of allowable dimensions, and thus the packing density to be achieved and, second, for reasons of an economical manufacture (see German OS 42 25 799).
Plate resonators are usually utilized as piezoactuators, a piezoelectric material, for example lead-zirconate-titinate (PZT), being provided between two metal electrodes. The carrier plate, which simultaneously serves as the diaphragm plate over the ink pressure chambers, for the piezoactuators can be composed of glass, ceramic, plastic or metal. In the latter instance, one electrode can be omitted; however, a conductive glue in then required.
The nature of the arrangement, application and contacting of the piezoactuators is thereby a critical problem.
German OS 37 10 654 discloses a planar ink printer head assembled of metal plates. One of the plates is a diaphragm plate of nickel having a plate thickness of 0.03 mm on which piezolaminae having a diameter of approximately 1 mm are arranged as drive elements for the pressure chambers in a number corresponding to the number of nozzles. The diaphragm plate is followed by a pressure chamber plate of nickel having a plate thickness of 0.2 mm; this corresponds to the desired height of the pressure chambers.
The piezolaminae are individually glued or soldered onto the diaphragm plate in the regions above the pressure chambers. The assembly and adjustment expenditures required therefor are substantial.
The conditions are also analogous in another known ink printer head, disclosed in U.S. Pat. No. 4,703,333. Here, the coatings of the piezoactuators facing away from the diaphragm plate are contacted to the terminals of a ribbon conductor.
German OS 38 05 279 further discloses a piezoelectric ink printer head having a solid-state piezoceramic body, this ink printer head having transducers arranged parallel side-by-side. Each transducer includes a planar, piezoelectric drive element, a pressure chamber, an ink channel and a nozzle. The pressure chambers, the ink channels and the nozzles are fashioned as cavities in a piezoceramic body. Each drive element has an outer electrode, an inner electrode and an active piezoceramic layer arranged between the electrodes. The drive elements are acoustically separated from one another by incisions in the active piezoceramic layer. The incisions are intended to prevent cross-talk between the individual drive elements. For manufacturing the solid-state piezoceramic member, piezoceramic green films are stacked on top of one another, pressed in a vacuum and sintered. A piezoceramic green foil is structured by etching; the cavities that have arisen correspond to the shape of the pressure chambers, the pressure chamber outlet as well as the ink channels. The etching ensues with spray etching or laser etching. An intermediate green foil of piezoceramic that is metallized on one side is placed onto the structured piezoceramic green foil. After the sintering, the piezoceramic of the intermediate green foil forms the pressure chamber walls, whereby the metallization lies at that side facing away from the pressure chambers. The metallization is produced by printing the intermediate green foil with a metal paste. After the sintering, it forms the inner electrodes connected to one another. An upper piezoceramic green foil is arranged on the intermediate green foil and the active piezoceramic layers arise therefrom after sintering.
After the sintering of the stacked and pressed piezoceramic green foils, the openings of the ink channels are uncovered by removing material on the basis of mechanical processing. The outer electrodes are applied onto the outer side of the active piezoceramic layers by sputtering using a mask or by silkscreening. This is followed by the polarization of the transducers and the separation of the drive elements. The piezoceramic member that has arisen in this way is contacted with terminals of a terminal ribbon and is introduced into a housing or a retainer frame.
In another known method for manufacturing a piezoceramic element for ink jet printers, see German OS 37 33 109, and that is likewise based on the sintering of piezoceramic green foils, sintering temperatures from 1100-1300.degree. C. in an oxygen atmosphere are required. Platinum or metals of the platinum group are utilized as electrode material that is suitable for the sintering process. These two latter solutions have the disadvantage that time-consuming and energy-consuming high-temperature processes and expensive electrode material are required. Moreover, only the finished, solid-state piezoceramic body can be polarized. The electrode material must be resistant to corrosion with respect to the ink since one electrode is accommodated in the ink chamber.
German OS 38 04 165 also discloses a method for equipping an ink jet print head with piezoactuators, whereby a piezoceramic plate is first firmly joined to a diaphragm plate and only then does a separation of the piezoactuators from the piezoceramic plate ensue. The diaphragm plate is composed of glass and the side thereof facing toward the piezoceramic plate provided with a zinc or nickel oxide layer. The two plates are joined with a glue. The piezoactuators are detached with a separating means such as a laser beam or a saw. In this way, the piezoceramic plate serves as an aid to assembly and prevent an incorrect polarization. The metal oxide layer on the glass plate constitutes the common electrode for the piezoactuators. A conductive glue is required so that a reliable contact is present between one side of the piezoactuator and the metal oxide layer. As is known, metal layers on a glass substrate have poor adhesion. Since the joint between the metallized glass diaphragm and the piezoactuators is subject to high mechanical loads due to the periodic oscillations, delamination of the metal layer from the glass can occur, and thus a failure of the printer module can result.